It is known that intake air is compressed by a supercharger such as a turbocharger or a mechanical supercharger and the compressed air is supplied to an engine (i.e. supercharging) as a technique for improving the output of the engine, and such method is widely used in the field of engines for vehicles, for example.
A supercharger includes a compressor rotational body including a rotational shaft and a compressor impeller mating with the rotational shaft on an end side of the rotational shaft. The compressor rotational body is configured so as to be rotated at high speed by e.g. a turbine impeller or an electric motor provided coaxially with the compressor rotational body.
Usually, a rotational shaft and a compressor impeller are separately manufactured and have their balance adjusted separately, and then they are assembled together into a compressor rotational body.
A compressor rotational body is assembled typically by means of a method called “clearance fit” (loose fit).
The clearance fit is a method where the outside diameter of the shaft is set to be smaller than the inside diameter of the hole which the shaft mates with. By this method, a small gap is formed between the rotational shaft and the compressor impeller, and thus, the compressor rotational body may be assembled with the center positions of the rotational shaft and the compressor impeller out of alignment to the extent of the size of the gap. If the compressor rotational body is assembled with the center positions of the two out of alignment, the center of gravity of the rotational body may not align with the center position, and accordingly, an eccentric load may applied to the compressor rotational body during rotation at high speed, which may cause breakage, abnormal noise, or the like. The misalignment between the center of gravity and the center position of the rotational body may be removed in the balance adjustment (processing) in a subsequent process. However, if the amount of misalignment is too large, the misalignment may not be removed by processing, and disassembling and reassembling may be necessary.
In order to solve the above problem, method called “interference fit” may be employed to assemble the rotational shaft and the compressor impeller. The interference fit is a method where the outside diameter of the shaft is set to be larger than the inside diameter of the hole which the shaft mates with. As the diameter of the shaft is larger than the diameter of the hole, press fitting, shrink fitting where the compressor impeller is heated, cooling fitting where the rotational shaft is cooled, or the like are employed for the assembly.
For example, Patent Document 1 discloses a technique where the outside diameter of a part of the rotational shaft is formed to have a slightly larger than the inside diameter of the insert hole of the compressor impeller, and the rotational shaft and the compressor impeller are assembled together through interference fit between the large-diameter part of the rotational shaft and the insert hole of the compressor impeller.
Patent Document 2 discloses a technique where the outside diameter of a part of a nut to be screwed on the rotational shaft on an end side of the rotational shaft is formed to have a slightly larger than the inside diameter of the insert hole of the impeller, and the rotational shaft and the compressor impeller are assembled together through interference fit between the large-diameter part of the nut and the insert hole of the impeller.